Viruses are microorganisms that depend, to some degree, on host cell components for their growth and replication. Viral infection and replication in host cells generally results in disease, whether the host is an animal or plant. Human diseases caused by viral infections include the acquired immunodeficiency syndrome (AIDS) and hepatitis. A general discussion of this field is presented in Fundamental Virology, Second Edition, (ed. B. N. Fields, D. M. Knipe, R. M. Chanock, M. S. Hirsh, J. L. Melnick, T. P. Monath, and B. Roizman, Raven Press, Ltd., New York, N.Y. 1991).
Retrovirus Replication
Retroviruses comprise a large family of viruses that primarily infect vertebrates. Many diseases, including the induction of some tumors, are associated with retroviral infection (see Fundamental Virology, supra, pp. 645-708). All retroviruses, regardless of their clinical manifestations, have related structures and modes of replication.
Retroviruses contain an RNA genome that is replicated through a DNA intermediate. Inside the cell, the viral genome serves as a template for the synthesis of a double-stranded deoxyribonucleic acid (DNA) molecule that subsequently integrates into the genome of the host cell. This integration occasionally results in the induction of a tumor in the infected host organism. Following integration, a complex sequence of events leads to the production of progeny virions which are released from the infected cell.
Early in the retroviral life cycle, the RNA genome is copied into DNA by the virally encoded reverse transcriptase (RT). This enzyme can use both RNA and DNA templates, thereby producing the first strand of DNA (the negative strand) from the infecting RNA genome and a complementary second strand (the positive strand) of DNA using the first DNA strand as a template. To synthesize these DNA strands, the RT utilizes cellular substrates called deoxynucleoside triphosphates (dNTP).
Human retroviruses can be grouped into the leukemia viruses (HTLV type viruses) and the immunodeficiency viruses (HIV type viruses). HTLV infection may lead to one form of leukemia. Acquired immunodeficiency syndrome (AIDS) is caused by a form of HIV, with HIV-1 being more virulent than HIV-2. Both HTLV and HIV infect peripheral blood lymphocytes (PBL).
HIV Infection
HIV-1 was first identified as the causative agent of AIDS in 1983. The AIDS pandemic is now one of the most serious health problems worldwide. Catastrophic medical and social consequences are likely to extend into the next century. The World Health Organization (WHO) has estimated that between eight and ten million people are currently infected with HIV, and that approximately ten times as many individuals will be affected in the next decade. The large pool of HIV carriers makes the development of effective antiviral treatments a medical priority.
The initial HIV-1 infection may occur without accompanying symptoms, but most of the patients experience an acute HIV syndrome within 2 to 6 weeks of exposure to the virus. This syndrome is characterized by fever, headaches, sore throat with pharyngitis, generalized lymphadenopathy and rashes. During this phase the virus is replicating abundantly and is detectable in the blood and the CD4+T-cell number falls from a normal amount of 1000/mm.sup.3 to about 500/mm.sup.3. Antibdies to HIV-1 proteins appear in the serum between 2-12 weeks after primary infection. The sequence of appearance of these antibodies can be followed by the Western blot test, which detects the serum antibodies that bind to specific viral proteins. A positive Western blot response to gp160, gp120, p65, p55, gp41, p32, p24 and p18 proteins demonstrates that antibodies to various HIV-1 proteins are being produced. The process of change from negative for all the proteins to positive for the entire set is referred to as seroconversion. It has recently been demonstrated that during seroconversion there is a high level of virus present in the blood. The cellular arm of the immune response is also activated during seroconversion. (Borrow et al. Nature Medicine 3:(2) 212-217, 1997; Goulder et al. Nature Medicine 3:(2) 205-211, 1997). Both humoral and cellular immune response together are associated with the decline of viral load in body fluids, or viremia, during acute primary infection. In the absence of antiviral therapy, the immune system can partially control viremia. When the viremia decreases in the blood, the CD4+T-cell number rises, but absent effective treatment, the T-cell population never fully recovers to the normal level.
Viral load, measured as HIV-1 RNA is the best available indicator of disease progression and reduced concentration of HIV-1 in various tissues and fluids in response to antiretroviral therapy is predictive of improved prognosis (Mellors, J. W. et al. Science 272(5265) 1167-1170, 1996).
Antiviral Therapies
There is a critical need to develop effective drug treatments to combat RT-dependent viruses such as HIV. Such efforts were recently urged in the United Kingdom-Irish-French Concorde Trial conclusions which reported that the nucleoside analog zidovudine (AZT), a mainstay in the treatment of patients infected with HIV-1, failed to improve the survival or disease progression in asymptomatic patients. Other nucleoside analogs such as 2',3'-dideoxyinosine (ddl) are currently under evaluation. The effects of ddl on disease progression and patient survival endpoints have not been adequately investigated. Non-competitive HIV-1 RT inhibitors and HIV-1 protease inhibitors have also been recently developed. These materials have different antiviral activities and pharmacokinetics properties, but they all directly target HIV-1 proteins. Despite the high efficacy of these compounds, the initial in vitrolin vivo testing has been characterized by the rapid onset of variants of HIV-1 resistant to these drugs. These drug-resistant variants, or escape mutants, retain their virulence, and appear to play a major role in the virus' ability to eventually overwhelm the human immune system. A peculiarity of HIV is that it demonstrates an extremely high rate of both reproduction and mutation. As a direct consequence, drugs which demonstrate what would in any other context be regarded as high efficacy (99.9% reduction of viral load in plasma) have not been shown to be able to eliminate the virus from an individual's system. Further, an individual may have undetectable levels of virus as measured by viral load in plasma and biopsy of lymph nodes during treatment, and yet remain infected: once treatment is stopped, the viral rate of replication increases, and the viral load rebounds. In an attempt to obtain greater accuracy, the present inventors have used the most sensitive test methods available. Further, testing of lymph nodes is done by extracting an entire node as opposed to a biopsy sample.
Since escape mutants play such a significant role in the development of the disease, a major focus in current efforts to find a mode of treatment for AIDS is to develop strategies that feature multiple, highly effective, concurrent attacks on HIV in an effort to completely eradicate the virus from an individual's system. The only conclusive proof of effectiveness will be lack of rebound of the viral load in the individual's tissues over time.
At present, there is much interest in trying various combinations of two, three or even four drugs simultaneously. However, it has been admitted that the number of "promising" drugs is "almost astronomical". See Antiviral Therapy for Human Immunodeficiency Virus Infections, E. De Clercq, Clinical Microbiology Reviews, 8:2, Am. Soc. for Microbiology (April 1995).
A triple drug combination involving the use of AZT, 3TC and protease inhibitors has been suggested for the treatment of HIV-1 infection and eradication of the virus. The efficacy of this combination is thought to originate from the potency of the protease inhibitors and the mechanism of action of the AZT/3TC combination in inhibiting the rebound of resistant mutants. However, neither the protease inhibitors nor 3TC easily penetrate to certain organs such as lymph nodes and the brain, and the combination of protease inhibitor, AZT and 3TC apparently does not completely eradicate HIV-1 in macrophages or in quiescent cells, which are major reservoirs of HIV-1. Further, patients who have interrupted therapy using AZT, 3TC and protease inhibitors and then rebounded cannot be treated as effectively with the same combination because they develop resistant mutants.
Hydroxyurea has been widely used over the last three decades for the treatment of leukemia, sickle cell anemia, and has more recently been suggested for use in the treatment of HIV infections, see Hydroxyurea as an Inhibitor of Human Immunodeficiency Virus-Type 1 Replication, F. Lori, et al., Science 266:801-805 (1994); possibly in combination with a nucleoside analog such as AZT, ddl, or ddC, although it has been admitted that clinical trials using hydroxyurea alone or in combination with nucleoside analogs will be essential to assess the actual impact of use of hydroxyurea in HIV-1 impacted patients. Hydroxyurea and AIDS: An Old Drug Finds a New Application? F. Lori and R. Gallo, Aids Research and Human Retroviruses Vol. 11, No. 10 Mary Ann Liebert, Inc. (1995). EPO patent publication 94918016.0 filed May 17, 1994 and corresponding to U.S. Ser. No. 08.065,814, filed May 21, 1993, which is incorporated herein as if set forth in full, describes the administration of hydroxyurea in combination with ddl, and has reported a therapeutic effect in that CD4+T-cell populations stabilized or increased in human volunteers. This result does not necessarily demonstrate that any of the individuals were cleared of the virus, because when any patient has stopped any therapy to date, an immediate rebound of viral load has occurred.
Hydroxyurea and nucleoside analogs such as ddl have potent effects on resting cells and macrophages (ref. Lori, PNAS 93 and Science 94; Goa-Wy; Agbaria R., Driscoll, J. S.,; Missuya, H.; J. Biol-Chem. Apr 29, 1994; 269(17); 12633-8; AU: Gao-W. Y.; Shirasaka, T.; Johns, D. G.; Broder, S.; Mitsuya, H.; J.Clin. Invest. 1993 May: 91(5): 2326-33) which one can speculate represents the route of initial infection during sexual, parenteral and vertical transmission, (1. SO: Science, Aug. 27, 1993:261(5125)1179-81; 2. SO: J. Clin. Invest. 1994 November: 94(5): 2060-7 4. SO: J. Clin. Microbiol. 1995 Feb; 33(2); 292-7, 5. S: AIDS. May; 9, 1995(5): 427-34; 6. SO: J. Exp. Med. Apr. 1, 1996; 183(4): 1851-6), and this could represent an advantage of the proposed combination.
Protease inhibitors have received much attention recently in the press as being useful in combination with other drugs such as nucleoside analogs, most especially the combination of AZT and 3TC, to inhibit HIV replication enough to yield improved quality of life for AIDS patients. It has been reported that the viral load in the plasma of such patients is greatly reduced, but not necessarily eliminated, and that whenever treatment has been stopped, the patients have experienced an increase in viral load (rebound) within a matter of 2-3 days.
The present invention is based on the discovery that a combination of hydroxyurea, a nucleoside analog, and a protease inhibitor can be used to inhibit HIV in human beings, with greatly improved results in that viral rebound may be delayed for at least three to eight weeks or more. These results indicate that the combination may be used for the treatment of HIV infection and eradication of the virus. Again, this combination takes advantage of the potency of the protease inhibitors, especially Indinavir. The HU/nucleoside analog combination has a different mechanism of action from that of the AZT/3TC combination. Further, it has been shown that the combination of HU and the nucleoside analog ddl is unable to prevent the onset of mutant viral strains conferring resistance to ddl, but the mutants are still sensitive to standard doses of ddl in the presence of HU. In addition, HU can easily penetrate to the organs such as lymph nodes and the brain, and can completely block the replication of HIV-1 in macrophages. Yet a further advantage is that viruses which are resistant to ddl and which have escaped can be inhibited by the addition of HU. Consequently, patients who have interrupted the treatment can be repeatedly treated effectively with the combination of HU, ddl and protease inhibitors.